This she did because she had seen something under it--a round knob which had been covered by the leaves hanging over it.

It was the knob of a door.
She put her hands under the leaves and began to pull and push them aside.

Thick as the ivy hung, it nearly all was a loose and swinging curtain, though some had crept over wood and iron.

Mary's heart began to thump and her hands to shake a little in her delight and excitement.

The robin kept singing and twittering away and tilting his head on one side, as if he were as excited as she was.